Aromatic polycarboxylic acids are conventionally produced by liquid phase catalytic oxidation of feedstocks containing a polymethyl substituted aromatic hydrocarbon, such as a xylene. Such liquid phase reaction systems are shown in U.S. Pat. Nos. 3,170,768 and 3,092,658, both to Baldwin. Because the chemical conversion of the polymethyl substituted aromatic reactant to the aromatic polycarboxylic acid product is known to be exothermic, reaction solvents are typically employed to dissipate the resultant heat of reaction in a reflux loop. The current practice is to produce the aromatic polycarboxylic acid product in a continuous process or system that includes an oxidation reactor equipped with a reflux system. The reactor contents include water, di or trimethyl substituted hydrocarbon reactants, reaction solvent, and a suitable oxidation catalyst for effecting conversion of the reactants to the desired polycarboxylic acid product. The oxidation reactor is also equipped with means for agitating the reactor contents.
In a conventional continuous oxidation process for producing aromatic dicarboxylic acid from a xylene, excess heat of reaction is typically removed via vaporization of the reaction solvents. More particularly, the solvent vaporization typically takes place in the oxidation reactor; and condensation of the vapors emanating from the reaction admixture typically takes place in a series of heat exchangers included within an overhead system. The heat exchangers, typically physically located above the oxidation reactor, allow condensed solvent to be refluxed to the oxidation reactor by gravity.
In one conventional process, the elevated heat exchangers are utilized to separate the water-containing and reactant-bearing solvent vapors into a water-rich aqueous liquid fraction and a reactant-enrich vapor stream. The water-rich aqueous liquid fraction is then removed from the elevated heat exchangers while the reactant-enriched vapor stream is further processed to recover vaporized reactant and solvent. Removal of the water-rich fraction in this manner reduces the concentration of water in the reactor-contained solvent which, in turn, lowers the concentration of oxidation catalyst that is required to effect the desired conversion reaction.
The above-mentioned overhead system includes steam-generating heat exchangers, water-cooled heat exchangers, gas-liquid separator equipped with means for venting non-condensible gases, and scrubbers. In the steam-generating overheat heat exchangers, which are generally of shell-and-tube construction, condensation of the vaporized reaction solvent takes place on the tube side while useful process steam is generated on the shell side. The water-cooled heat exchangers, typically located down stream of the steam-generating exchangers, are utilized for the purpose of assisting in the condensation of the vaporized solvent before the condensed solvent is returned to the oxidation reactor, generally as reflux. Effluent from the water-cooled exchangers is passed to the gas-liquid separators. Condensed liquid, recovered via the separators, is returned to the oxidation reactor. Residual reactant-bearing solvent vapors as well as non-condensible gases are vented from the separators and passed through the scrubbers which serve to further recover unreacted reactant and solvent from the non-condensible gas-bearing vapors before such vapors are vented. In certain situations, such vapors are passed through energy-recovery devices, such as an expansion turbine, before venting.
Many conventional oxidation reactors were originally designed to operate at a predetermined temperature range. For a variety of reasons, including product quality, it has become desirable to reduce the reaction temperature to below the temperature ranges previously utilized for the oxidation reaction.
For example, reduced reaction temperatures tend to reduce undesirable burning losses of the polymethyl aromatic reactant as well as the solvent. Reduced reaction temperatures have also been observed to result in a reduction of undesirable oxidation reaction by-products. Thus, it is desirable to reduce the process temperature range so as to improve product yields and quality while reducing operating costs of the process.
In the conventional polymethyl aromatic oxidation process, lower reaction temperatures require a simultaneous reduction in the reactor operating pressure. However, as the reactor pressure is reduced, vapor velocities in the reactor increase with attendant reduction in reactor liquid phase residence times. Pressure drops in overhead piping and heat exchanges increase as well. Consequently, as the reactor temperatures are lowered in a conventional polymethyl aromatic oxidation process, equipment limitations are encountered which require either a reduction in unit throughput or significant capital expenditures for equipment alterations needed to maintain capacity.
Moreover, as the system total pressure is reduced in a conventional process to achieve the desired lower temperature, the oxygen partial pressure at a given dry basis vent oxygen content is also reduced. This imposes an undesirable limitation on operable oxygen partial pressures.
Lower reaction temperatures in the conventional process also reduce the available temperature differential for generating steam at a useful pressure in the reactor overhead hot condensers.
Accordingly, it would be desirable to provide an improved polymethyl aromatic oxidation process that can be operated at relatively lower process temperatures while obviating, or at least minimizing, the aforementioned difficulties when the conventional aromatic alkyl oxidation process is operated at such relatively lower process temperatures. The present improved process satisfies the foregoing desires.